CSIRO study found forest fires linked to climate change: 9 out of the 11 fire years, each with more than 500,000 km2 burned, occurred since 2000.

And the bad news for rural and regional Australia just keeps on coming.....

‘Lead author and CSIRO chief climate research scientist Pep Canadell said the study established the correlation between the Forest Fire Danger Index – which measures weather-related vegetation dryness, air temperature, wind speed and humidity – and the rise in area of forest burned since the 1930s.

“It’s so tight, it’s so strong that clearly when we have these big fire events, they’re run by the climate and the weather,” Dr Canadell said…

“Almost regardless of what we do the overall extent of the fire, really, is dictated by those climate conditions,” he said.

Climate scientists have found climate change is exacerbating the key fire risk factors identified by CSIRO’s study, with south-eastern Australia becoming hotter, drier and, in a particularly worrying trend, more prone to high wind on extremely hot and dry summer days.

The weather system that drove a blast furnace’s worth of westerly wind across NSW and Victoria’s forests, sparking some of the worst fires of the Black Summer in 2019-20, will be up to four times more likely to occur under forecast levels of global warming.’ [The Sydney Morning Herald, 26 November 2021]

Nature.com, Nature Communications, 26 November 2021, article excerpt:

“Multi-decadal increase of forest burned area in Australia is linked to climate change”

Josep G. Canadell, C. P. (Mick) Meyer, Garry D. Cook, Andrew Dowdy, Peter R. Briggs, Jürgen Knauer, Acacia Pepler & Vanessa Haverd

Received 24 July 2020

Accepted 03 November 2021

Published 26 November 2021

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ABSTRACT

Fire activity in Australia is strongly affected by high inter-annual climate variability and extremes. Through changes in the climate, anthropogenic climate change has the potential to alter fire dynamics. Here we compile satellite (19 and 32 years) and ground-based (90 years) burned area datasets, climate and weather observations, and simulated fuel loads for Australian forests. Burned area in Australia’s forests shows a linear positive annual trend but an exponential increase during autumn and winter. The mean number of years since the last fire has decreased consecutively in each of the past four decades, while the frequency of forest megafire years (>1 Mha burned) has markedly increased since 2000. The increase in forest burned area is consistent with increasingly more dangerous fire weather conditions, increased risk factors associated with pyroconvection, including fire-generated thunderstorms, and increased ignitions from dry lightning, all associated to varying degrees with anthropogenic climate change.

INTRODUCTION

The extraordinary forest fires in Australia in 2019 and 20201 have brought further interest in detecting changes in fire activity, the possible role of anthropogenic climate change and their likely future trends both in Australia and globally ^2,3,4,5,6.

Terrestrial ecosystems in Australia are among the most fire prone in the world, with fire regimes varying widely ^7,8. Fire activity is dominated by savanna and rangeland fires in the northern and western parts of the continent characterized by fire return intervals of less than 5 years ^7,9. Forests in the east and south have fire return times of decades to more than a century, with subtropical and tropical forests in the northeast burning rarely or not at all ⁹. Fire, including cultural burns by indigenous people, has shaped the function and structure of most Australian ecosystems for millennia ^10,11.

Against this background of fire activity, Australia’s mean temperature has increased by 1.4 °C since 1910 with a rapid increase in extreme heat events, while rainfall has declined in the southern and eastern regions of the continent, particularly during the cool half of the year ^12,13,14. These changes can affect the four components that must simultaneously come together for fire to occur: biomass production, its availability to burn (fuel loads), fire weather, and ignition ⁷, making Australian forests vulnerable and sensitive to changes in fire activity.

Previous studies showed increased fire danger due to changes in weather conditions over past decades in Australia ^5,15,16, climate change fingerprinting to individual fire events and trends ^17,18,19, and predicted increases in fire danger under future climate change due to increasing atmospheric concentrations of greenhouse gases ^2,20,21. Although these studies indicate more dangerous weather conditions for wildfires in a warmer world, studies also suggest that trends due to climate change might not be clearly detectable until later in the coming decades owing to the high natural variability and extremes of the Australian climate ^4,22,23,24.

Fuel loads and trends, as effected by climate, human activity and time since the last disturbance, also play a role in determining fire risk ^25,26. This link is a central motivation for using prescribed burning to reduce fuel availability ²⁷, which in Australia is managed through changes in the frequency of prescribed burns ²⁸. Although there is some debate on their value to reduce fire risk ²⁹, particularly during extreme fire weather conditions ^2,30, fuel loads and their distribution and structure are key determinants of fire spread, intensity and severity ⁷.

Here we analyze trends of the burned area in forest ecosystems in Australia, which are dominated by temperate forests extending over the southern and eastern regions of the continent. We use a high-resolution (1.1 km x 1.1 km) burned area satellite record available based on NOAA-AVHRR (32 years), the NASA-MODIS burned area at 500 m resolution (19 years), and the fire histories from State and Territory government agencies (90 years). In addition, we analyze trends of nine wildfire risk factors and indices that relate to characteristics of fuel loads, fire weather, extreme fire behaviour, and ignition, which together with the burned area enable us to infer the causal influence of climate change on fire activity.

RESULTS

Trends in area burned

At a continental scale, total annual burned area (fire year defined as July to June to include the Austral summer of December to February) using the NOAA-AVHRR dataset (“Methods”: Burned area data), significantly increased over the past 32 years albeit with large interannual variability (Fig. 1a; Linear fit, p value = 0.04, Supplementary Table 1). The high variability is in part driven by large-scale modes of atmospheric and oceanic variability such as El Niño Southern Oscillation (ENSO) and the Southern Annual Mode ^31,32 that influence fire weather conditions ^16,22. Nine out of the 11 fire years, each with more than 500,000 km2 (>50 Mha) burned, occurred since 2000.

Forest ecosystems also show increased burned area over time (Fig. 1b, linear fit, p value = 0.02, Supplementary Table 1; Fig. 2). The increasing trend is statistically significant with and without the 2019 fire year, indicating a robust increasing trend even before the extraordinary large burned area of that year (Supplementary Table 1). Forests in Australia experienced an annual average increase of 350% in burned area between the first (1988-2001) and second (2002-2018) half of the record, and an increase of 800% when including 2019. The 2019 fire year burned about three times (60,345 km2) the area of any previous year in the 32-year AVHRR-Landgate record (Fig. 3, Supplementary Fig. 1, “Methods”: Burned area). The burned area of the 2019 fire year was estimated at 71,772 km2 based on State and Territory agencies (NIAFED) and 54,852 km2 based on NASA-MODIS, with an average for the three products of 62,323 ± 8,631. Ten out of eleven fire years with at least 5000 km2 (>0.5 Mha) burned have occurred since 2001. These trends are broadly consistent across the three burned area products (Supplementary Fig. 1).

Fig. 2: Monthly burned forest area for fire years (July to June).